This has happened on two occasions with my 17 months old Sony RDR - HXD 710. The second time yesterday. The machine froze so I couldn't use the handset (new batteries were inserted recently) or switch off the machine itself. So I had to switch off the power, unplug, wait 10 seconds before plugging in and switching on. The "WELCOME" sign starts flashing and after a few minutes "RECOVERY" flashes. Wait until this last process finishes and if like mine, hey presto it works again! Seems to be like a computer - it gets a little tired working and has to be re-booted! Hope this works for you.
Brian Curry aka Mr Boo, April 2007